https://blog.csdn.net/weixin_40676873/article/details/81166139
https://blog.csdn.net/lanshuizhiyun/article/details/77337453
https://www.cnblogs.com/yangqingli/p/4925011.html
NOIp常用的算法
原文：https://www.cnblogs.com/littlehb/p/11888533.html
